1. Field of the Invention
The invention relates to a cooler, e.g. an oil cooler, composed of extruded shapes comprising a circumferentially closed inner core concentrically mounted within an outer circumferentially closed shell.
2. Description of the Prior Art
A cooler of this type is disclosed in U.S. Pat. No. 4,345,644. The inner and the outer shells in this cooler are provided as concentric walls in a one-piece coextruded shape where an annular passage formed between the shells is divided into individual passageways or chambers by partitions extending radially from the shells. The open front ends are closed by circular caps provided with inlet and outlet apertures for the cooling medium. These caps are attached to the extruded shape by a plurality of screws. Manufacture of an extrusion die to provide such a shape is a difficult task. Furthermore, the ratio between the passageway cross-section and its perimeter is unfavourable. This ratio decides the cooling capacity or performance of the cooler. Another parameter having quite important influence on the cooling capacity is the thickness of the medium layer between the two adjacent limiting surfaces of the cooler. Extensive tests show that the capacity increases substantially when this layer thickness is within double the value of the friction or boundary layer thickness of the cooling medium. The boundary layer is defined as a flow layer of a gaseous or liquid medium formed along a fixed wall in its vicinity and where the flow velocity increases from zero asymptotic to the velocity of the outer flow. The boundary layer thickness is then the distance from the wall when the flow velocity achieved is approximately 99% of the outer velocity. These values are known for liquid and gas media. For air and water these values represent about two millimeters, for oil (machine oils) the value can reach up to 6 millimeters. It is therefore important for an efficient cooler that the distance between the cooling surfaces is relatively small. On the other hand the cooler construction must be designed in such a way that the cooler, while complying with the above mentioned criteria, is also adaptable to different flow rates without changing the optimal flow velocity of the media which likewise represents a factor influencing the cooling capacity.